Laser-Driven Recollisions under the Coulomb Barrier.

Photoelectron spectra obtained from the ab initio solution of the time-dependent Schrödinger equation can be in striking disagreement with predictions by the strong-field approximation (SFA), not only at low energy but also around twice the ponderomotive energy where the transition from the direct to the rescattered electrons is expected. In fact, the relative enhancement of the ionization probability compared to the SFA in this regime can be several orders of magnitude. We show for which laser and target parameters such an enhancement occurs and for which the SFA prediction is qualitatively good. The enhancement is analyzed in terms of the Coulomb-corrected action along analytic quantum orbits in the complex-time plane, taking soft recollisions under the Coulomb barrier into account. These recollisions in complex time and space prevent a separation into sub-barrier motion up to the "tunnel exit" and subsequent classical dynamics. Instead, the entire quantum path up to the detector determines the ionization probability.

Interference carpets in above-threshold ionization: from the Coulomb-free to the Coulomb-dominated regime.

The velocity map recorded in above-threshold ionization of xenon at 800 nm exhibits a distinct carpetlike pattern of maxima and minima for emission in the direction approximately perpendicular to the laser polarization. The pattern is well reproduced by a numerical solution of the time-dependent Schrödinger equation. In terms of the simple-man model and the strong-field approximation, it is explained by the constructive and destructive interference of the contribution of the long and the short orbit. Strictly perpendicular emission is caused by ionization at the two peaks of the laser field per cycle, which results in a 2hω separation of the above-threshold ionization rings.

Time-resolved holography with photoelectrons.

Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.

Low-energy structures in strong field ionization revealed by quantum orbits.

Experiments on atoms in intense laser pulses and the corresponding exact ab initio solutions of the time-dependent Schrödinger equation (TDSE) yield photoelectron spectra with low-energy features that are not reproduced by the otherwise successful work horse of strong field laser physics: the "strong field approximation" (SFA). In the semiclassical limit, the SFA possesses an appealing interpretation in terms of interfering quantum trajectories. It is shown that a conceptually simple extension towards the inclusion of Coulomb effects yields very good agreement with exact TDSE results. Moreover, the Coulomb quantum orbits allow for a physically intuitive interpretation and detailed analysis of all low-energy features in the semiclassical regime, in particular, the recently discovered "low-energy structure" [C. I. Blaga, Nature Phys. 5, 335 (2009) and W. Quan, Phys. Rev. Lett. 103, 093001 (2009).

Strong field ionization rate for arbitrary laser frequencies.

A simple, analytical, nonrelativistic ionization rate formula for atoms and positive ions in intense ultraviolet and x-ray electromagnetic fields is derived. The rate is valid at arbitrary values of the Keldysh parameter and confirmed by results from ab initio numerical solutions of the single active electron, time-dependent Schrödinger equation. The proposed rate is particularly relevant for experiments employing the new free electron laser sources.

Two-dimensional streaking: complete characterization of an arbitrarily polarized few-cycle laser pulse using a stereodetector technique.

We discuss the feasibility of measuring the temporal variation of the electric-field strength of a few-cycle laser pulse with arbitrary polarization using the attosecond streaking method. It is shown that a full characterization of the field requires measuring the photoelectron momenta in two opposite directions in the laser polarization plane for various delays of the extreme ultraviolet burst with respect to the probed laser pulse.

Coulomb asymmetry in above-threshold ionization.

A new method for including effects of the Coulomb potential in strong-field laser atom interaction is presented. The model is tested by comparing its results with experimental data of energy resolved angular distributions of photoelectrons. For elliptical polarization these exhibit a strong asymmetry. Our theory shows that this strong asymmetry for the low-energy electrons is induced by a small Coulomb force acting on the tunneling electron just after the exit of the tunnel. This is in contrast to the situation for high electron energies where the asymmetry arises via rescattering by the parent ion.

Laser-induced recollision phenomena: interference resonances at channel closings.

An analytical theory of the resonancelike phenomena in high-order above-threshold ionization is presented that explains details of the experimental spectra and theoretical simulations. It traces the observed features to the constructive interference of "quantum orbits" with long travel times at laser intensities where the N-photon ionization channels close. Characteristic differences show up between even and odd N. The effects are generic to all laser-induced recollision phenomena. For nonsequential double ionization, their signature in the momentum distribution of the final electrons is identified.

Delay-dependent amplification of a probe pulse via stimulated Rayleigh scattering.

Stimulated Rayleigh scattering of pump and probe light pulses of close carrier frequencies is considered. A nonzero time delay between the two pulses is shown to give rise to amplification of the delayed (probe) pulse accompanied by attenuation of the pump, both on resonance and off resonance. In either case, phase-matching effects are shown to provide a sufficiently large gain, which can exceed significantly direct one-photon absorption losses.